Episodic closure of the tidal inlet at the mouth of the Russian River — A small bar-built estuary in California Dane K. Behrens a, b , Fabián A. Bombardelli a, ⁎, John L. Largier b, d , Elinor Twohy c a Department of Civil and Environmental Engineering, University of California, Davis, One Shields Avenue, 2001 Ghausi Hall, Davis, CA 95616, USA b Bodega Marine Laboratory, University of California, Davis, 2099 Westside Road, Bodega Bay, CA 94923–0247, USA c Environmentalist, Burke Ave., Jenner, CA 95450, USA d Department of Environmental Science and Policy, University of California, Davis, One Shields Ave., Davis, CA 95616, USA abstract article info Article history: Received 23 March 2012 Received in revised form 11 January 2013 Accepted 15 January 2013 Available online xxxx Keywords: Tidal inlet Inlet closure Inlet morphology Bar-built estuary Bar-built estuaries with relatively small (cross sectional area b 100 m 2 ) and shallow tidal inlets are wide- spread in Mediterranean climates and along wave-exposed coasts. While similarly important to coastal sediment balances and estuarine ecosystems and more numerous than larger inlet systems, they suffer from a relative lack of understanding. This is especially true regarding the process of inlet closure, when the channel is filled with sediment deposited by wave-driven processes. Mouth closure is of growing concern owing to changes in inlet behavior in many systems in response to human influences. The use of many existing conceptual and quantitative models for closure is precluded by rapid morphological change, unsteady freshwater inputs, and difficulties in scaling well-established inlet relationships in these smaller systems. In this paper, over 60 years of daily closure records are analyzed for the Russian River mouth, a prototypical bar-built system in northern California. We show that the time-dependent closure pattern can be explained by an interaction of processes that act to scour the inlet (driven by tides and river) with those that act to deposit sand in the inlet (driven by waves). Tidal, weather-related, seasonal, and interannual cycles are observed. We improve an existing parametric model to evaluate these separate influences and show that it works well for predicting closure events at both the tidal and seasonal scales. Finally, we compare our data with closure records from the nineteenth and twentieth centuries to show that inlet closures have become shorter and more sporadic over time at this site and to identify anthropogenic drivers of this observed change. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Tidal inlets are widespread features on the coastlines of the world (Ranasinghe and Pattiaratchi, 2003). They exhibit a range of behaviors that depend on their degree of tidal and fluvial influence, as well as their size, shape, and stratigraphy. Small inlets (cross sectional area b 100 m 2 ) are a prominent subset that are commonly associated with ‘bar-built estuaries’ (where waves build a sandbar across the mouth of the estuary), especially along active continental margins in Mediterranean climates, such as in California (e.g., Webb et al., 1991; Behrens et al., 2009), Chile (Dussaillant et al., 2009), South Africa (Cooper, 2001; Smakhtin, 2004), and Australia (Ranasinghe and Pattiaratchi, 1998, 1999a, 2000; Gale et al., 2006, 2007; Rustomji, 2007). A hallmark of these systems is their intermittent connectivity with the ocean, as waves periodically block the inlet channel with sed- iment. Despite this variability, these systems are vital for many species that have adapted to and thus take advantage of the closed inlet condi- tions (Becker et al., 2009). In western North America, the state of the inlet is increasingly recognized as a critical factor for endangered salmo- nid populations, which migrate into the estuary and upriver to spawn in seasons when the inlet is typically open, but may also use the lower estuary as a nursery when the inlet is closed during the dry season (Hayes et al., 2008). In addition to preventing navigation and fish pas- sage between the ocean and the lagoon, inlet closures prevent tides from entering the estuary, which can have significant impacts on estu- arine mixing, circulation and flushing (e.g., Ranasinghe et al., 1999) — as well as negative impacts on water quality in the presence of pollutant loading. Small inlet systems are also important in land–ocean sediment transport as they are typical of the mouths of rivers originating in high-gradient catchments, which are the dominant contributors of sed- iment to the world oceans (Syvitski et al., 2003). Inlet morphology is governed by a balance between sediment import through wave-driven transport (constructive processes) and sediment export through tides and river flow (destructive processes) (e.g., FitzGerald, 1996). A state of dynamic equilibrium occurs when these transport rates balance, but this is rarely achieved on time scales at which morphology changes in these small inlets. At this scale, channel-bed friction constrains the flushing capacity of tides and river flow (e.g., Byrne et al., 1980). The resulting imbalance in sediment imports and exports may lead to changes in cross sectional Geomorphology xxx (2013) xxx–xxx ⁎ Corresponding author at: 2001 Ghausi Hall, One Shields Ave, Davis, CA 95616, USA. Tel.: +1 11 1 530 752 0949; fax: +1 11 530 752 7872. E-mail address: fabianbombardelli2@gmail.com (F.A. Bombardelli). GEOMOR-04242; No of Pages 15 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.01.017 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Please cite this article as: Behrens, D.K., et al., Episodic closure of the tidal inlet at the mouth of the Russian River — A small bar-built estuary in California, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.017